It is known that the thermal efficiency of gas turbines and combined cycle gas turbine-Rankine cycle engines is significantly reduced when they are operating at reduced loads. This reduction of efficiency is particularly evident when a constant drive speed is required, such as with electric generator service.
Various mechanisms have been applied to gas turbines to reduce the part-power efficiency reduction, such as multiple rotors, variable flow path geometry, and cycle regeneration. U.S. Pat. Nos. 3,150,487 and 3,703,807 have attempted to improve part-power efficiency of combined cycle gas turbine with exhaust heat-driven steam Rankine bottoming cycle powerplants by selectively heating the compressor inlet air as the power level is reduced. In U.S. Pat. No. 3,150,487, the inlet air is heated by a preheater supplied with low pressure steam extracted from the Rankine vapor generator, or by direct mixing of a portion of the gas turbine exhaust gases with the compressor inlet air. In the latter case, the gases are extracted upstream of the steam vapor generator. In U.S. Pat. No. 3,703,807, a portion of the exhaust gases is recirculated to the compressor inlet in the same manner, except the recirculated products are extracted from the exhaust stack downstream of the vapor generator.
An object of the present invention is to provide a combined cycle powerplant which operates at high efficiency under part load conditions, and is an improvement over known powerplants. A further objective is to provide a means of improving the part load efficiency of a combined cycle gas turbine engine when operating at a constant speed and varying load. Still further objectives and advantages of the invention will become apparent upon consideration of the present disclosure and claims in view of the accompanying drawings.